Sources of error in sorption and density measurements

In sorption measurements, volumetric or gravimetric procedures are commonly used to determine the amount adsorbed. At low pressures, thermomolecular f low and pressure differences according to Knudsen's law disturb measurement s. In volumetry, calibration of the dead space is required; in gravimetry, the influence of buoyancy has to be taken into account. In both cases, adso rption of the calibrating gas, usually helium, may disturb the measurements [1]. From the calibration measurements, the density of the sample can in p rinciple be calculated. However, it has been observed in many experiments t hat its value depends on the calibrating gas.